Process for injection molding of thermoplastics and split mold for effecting same

ABSTRACT

An injection molding process is adaptable to various branches of industry for the production of high-strength textured articles from thermoplastic materials. The process consists of successively filling the intercommunicating cavities of a split mold with a polymer melt, holding the latter under pressure, and cooling the melt. As the hot melt is held under pressure and cooled, it is forced through the mold cavities at a temperature not exceeding the polymer melting or yield temperature by not more than 40° to 50° C., and under a pressure of 900 to 3,000 kgf/cm 2 . The mold surfaces are cooled within the temperature range starting from -20° C. and ending with the polymer initial melting or yield temperature. Mounted in the split mold, having half-molds with a die and plunger, are conventional heaters arranged at the inlet and outlet openings in isolation from the remaining part of each half-mold.

The present invention relates to a process of injection molding of thermoplastics and a split mold for carrying this process into effect.

FIELD OF THE INVENTION

The invention is applicable for use in the machine-building industry and other branches of industry engaged in the manufacture of high-strength textured items from thermoplastic materials.

DESCRIPTION OF THE PRIOR ART

U.S. Pat. No. 3,374,304 discloses a process for injection molding of thermoplastics, according to which a polymer melt is fed under pressure through a gating system into the mold cavities arranged in parallel relationship with one another, whereupon the melt is held under pressure and then cooled. A first portion of the melt is initially forced into the first cavity of the mold to be cooled therein, then into the core of the molding being cooled in the first cavity, while a second portion of the melt is concurrently injected into the second cavity of the mold. The primary object of the process in question is to compensate for the shrinkage of the molding material by injection of the first and second portions of the melt, during which the outer layers of the molding drawn-in during cooling toward the center after the first injection, are pressed out toward the forming surface of the mold.

However, it is possible to carry out the injection of the melt only in a case where the mold cavities are designed for forming sufficiently thick items, for example, over 4 mm in thickness. It is necessary that the melt temperature should exceed by more than 50° C. the polymer initial melting- or yield temperature, and should exceed the mold temperature by not less than 60° C.

In the prior-art process, the mold cavity is filled periodically during repeated injections of the melt. In the period between injection cycles, the structural elements in the mold cavities are disoriented due to relaxation and isotropic structure is formed in the solidified layers of the preceding portion of the melt, whereas the oriented structure is produced at the boundary between the preceding and succeeding portions of the material. As a result, the molding has heterogeneous and poorly oriented structure in cross section, which renders impossible the production of moldings with good strength properties. It is common knowledge that the polymers with non-oriented structures have, as a rule, low strength characteristics. For example, for various grades of high-density polyethylene, the yield point is 220 to 260 kgf/cm², and the ultimate strength is 200 to 230 kgf/cm².

There is also known a process for injection molding of thermoplastics, which comprises feeding a polymer melt under pressure through a gating system into the mold cavities intercommunicating through openings. According to this process, a single mold can be used for the production of an item made up of two or more concentrically arranged parts (see, for example, French Pat. No. 1,165,678 and Germany, Federal Republic of, laid-open Application No. 2,030,701 cl. 39a 41/00, 1971).

The above-mentioned process allows for the production of moldings having superior strength characteristics as compared to those manufactured in accordance with U.S. Pat. No. 3,374,304. This can be explained by the fact that with the mold cavities being arranged in series the time during which the material in the first cavity is displaced during its flow tends to increase, this being the reason for incomplete orientation processes which occur under given temperature conditions of the process.

However, the production techniques described above are primarily designed to improve quality of series-arranged items or parts of the same item made in a single mold, while the problems of improving strength characteristics of the molding material are neither posed nor solved. To this end, the melt and the mold are brought to high temperatures. For example, for high-density polyethylene, the temperature is 240° C. and 90° C., respectively. It has been found that the above-mentioned temperature conditions of the process tend to increase the polymer chain mobility and, consequently, descrease the amount of oriented structural elements throughout the volume of the molding being produced.

It is an object of the present invention to provide a process for injection molding of thermoplastics, which will make it possible to substantially improve strength characteristics of thermoplastic materials, for example, to increase tensile strength thereof 2 to 3 times by creating appropriate conditions for directionally oriented structure to take place over the entire volume of the molding material.

Another object of the invention is to provide a split mold for carrying out this process.

These and other objects of the invention are accomplished by the provision of a process for injection molding of thermoplastics, comprising the steps of successively filling intercommunicating cavities of a split mold with a polymer melt, holding the latter under pressure and cooling the melt, wherein, after filling the cavities with the polymer melt and in the course of holding the latter under pressure and its cooling, the melt is forced through the mold cavities at a temperature not exceeding the polymer melting or yield temperature by not more than 40° to 50°, and under a pressure of 900 to 3,000 kgf/cm² until the polymer acquires a solid state throughout the volume of a thermoplastic article being produced, the mold surfaces being cooled within the temperature range starting from -20° C. and ending with the polymer initial melting or yield temperature, and wherein with an increase in pressure, the flow area of the outlet openings of the cavities is decreased and the temperature of the mold surfaces is raised within the aforeindicated temperature range.

As the polymer melt is forced through the mold cavities under a pressure of 900 to 1,400 kgf/cm², the mold surfaces are preferably cooled down to a temperature ranging from -20° C. to +60° C.

Moreover, the hot melt is preferably subjected to additional heating at the places of its transistion from one cavity to another.

In the course of pressing the melt through the cavities, at least one of the mold surfaces of a split mold is preferably displaced by force in the direction of the polymer flow until the polymer acquires a solid state throughout the volume of the molding being produced.

In addition, the polymer melt can be forced through the cavities by bringing together the forming surfaces of a split mold, with the dimension of the prefilled mold cavities being 1.5 to 3 times that of a finished item, while an excessive amount of the material is concurrently pressed out; the mold surfaces are brought together before the polymer acquires a solid state throughout the volume of the item being produced.

According to the invention, the polymer melt is forced through the cavities of the split mold with the ratio of the inlet opening flow area to the maximum distance between the mold surfaces being 0.2 to 4, and with the ratio of the outlet opening flow area to the cavity cross-section at the place of minimum distance between the mold surfaces in the direction normal to the polymer flow being 0.01 to 1.

It has been found that the process of crystallization proceeding in the articles produced in accordance with the injection molding technique of the invention is accompanied by the formation of extended-chain crystallites in polymers, which, by virtue of their chemical composition, are capable of crystallizing; or else, an ordered molecular orientation may take place in noncrystallizable thermoplastics due to the macromolecular extension occurring at the solid polymer-melt interface when the cavity is filled with the melt or in the course of shear at high stresses brought about by high pressures and low velocities of the flow at a time when the polymer melt is continuously forced through the mold cavities and the heated section through which the polymer melt passes on its way to the next cavity, as well as due to forced rotation of one of the forming surfaces until complete solidification takes place throughout the volume of the molding material.

In this way it becomes possible to improve strength properties of thermoplastic materials.

Furthermore, strength properties of the molding produced by the process of the invention are ensured by a uniform distribution of the external force over the solidification front and due to the formation of molecular and permolecular structure of the material, which is homogeneous over the molding cross-section and displays a maximum elongation in the direction of molding.

The process of the invention is preferably carried into effect by means of a split mold made up of two half-molds having coolant supply passages, said half-molds being mounted on plates of an injection-molding machine and carrying respectively a die and a force plunger, the forming surfaces of which define cavities with the halfmolds brought together, said cavities communicating with one another and with a gating system through inlet and outlet openings, wherein, according to the invention, conventional heaters are mounted in the die and in the plunger and located at the inlet and outlet openings in isolation from the remaining part of each half-mold, and wherein replaceable inserts are also provided at the inlet openings to alter the flow area of these openings.

It is preferable that the die cavity, at least the first as viewed in the direction of the polymer flow, be formed with a channel extending throughout its length and opened from the side of the cavity to receive an elongated rod member having a projection on its free end mounted therein for reciprocation, the rod member having its external surface, presented to the cavity, functioning as a forming surface with a longitudinally extending depression provided therein.

Where hollow moldings are to be produced, a power-driven forming mandrel is preferably mounted for rotation in the cavities of each half-mold in coaxial arrangement therewith, the mandrel carrying a movable thrust member connected with the half-molds by means of a lever mechanism.

The split mold may be variously otherwise embodied such that at least one of the forming surfaces of a die or a plunger is made movable in the direction of another surface so as to alter the volume of the cavity they form by means of a plunger incorporated in a power cylinder mounted in one of the half-molds, with a valve device of any conventional type being arranged in close proximity with the outlet opening to close the latter when required.

The invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 shows schematically the process molding two articles inter connected by a link member, according to the invention;

FIG. 2 shows a split mould for producing articles in accordance with the process of the invention, with a partial cross sectional view taken along a die and a plunger;

FIG. 3 is a cross section III--III of FIG. 2;

FIG. 3a is an embodiment of the rod member shown in FIG. 3;

FIG. 4 is a longitudinal sectional view of a die mold for the production of hollow articles in accordance with the process of the invention; and

FIG. 5 is a longitudinal sectional view of a die mold in which ione of the forming surfaces of the die is made movable toward the forming surface of the plunger.

The process according to the invention for injection molding of thermoplastics is exemplified by way of describing the production of a molding (FIG. 1) made up of at least two pieces 1 and 2 interconnected by a link member 3. These two items or pieces 1 and 2 are produced strengthened and textured in accordance with the process of the invention as distinct from the prior-art process which permits only the piece 1 to be strengthened while the piece 2 remains unstrengthened.

The following technological operations are used for the production of the strengthened pieces 1 and 2. First, intercommunicating cavities 4 and 5 of a split mold 6 are filled with a polymer melt, then the latter is held under pressure and cooled. As the polymer melt is held under pressure and cooled, it is forced through the cavities 4 and 5 of the split mold 6 at a temperature not exceeding the polymer melting or yield temperature by any more than 40° to 50° C. and under a pressure of 900 to 3,000 kgf/cm² until the polymer acquires a solid state throughout the volume of the article being produced. The forming surfaces of the mold 6 are cooled within the temperature range between -20° C. and the polymer initial melting or yield temperature. With an increase in pressure, the flow area of the outlet opening, communicating the cavities 4 and 5 of the mold 6, is descreased and the temperature of the mold surfaces is raised within the aforementioned temperature range.

The following factors are taken into account to ensure effective production of thermoplastic items. For example, if the polymer melt is forced through under a pressure of 900 to 1,400 kgf/cm², then the mold surfaces are cooled down to a temperature ranging from -20° C. to +60° C.

To prevent solidification of the polymer featuring narrow interval of the melting and yield temperature during its passage from one cavity to another through an opening or a channel intercommunicating these cavities, the melt is additionally heated at the places of its transition from one cavity to another.

As the melt is pressed through the cavities 4 and 5, at least one of the forming surfaces of the mold, shown in FIGS. 2 and 3, can be displaced by force in the direction of the polymer flow until the polymer acquires a solid state throughout the volume of the molding being produced.

The polymer melt can be forced through the cavities for example, either by advancing the screw conveyer of an injection-molding machine or by bringing together the forming surfaces of the split mold, such as shown in FIG. 5, with the dimension of the prefilled mold cavities being 1.5 to 3 times that of finished items. Simultaneously, an excessive amount of the material is pressed out from the cavities in question. Also, the mold surfaces are brought together before the polymer acquires a solid state throughout the volume of the item being produced.

The polymer melt is effectively forced through the cavities 4 and 5 of the split mold 6 (FIG. 1), in a manner similar to that described in relation to the split molds shown in FIGS. 2, 4 and 5, with the ratio of the inlet opening flow area R_(I) and R₂ to the maximum distances between the forming surfaces of the cavity 4 being 0.2 to 4, and with the ratio of the cavity outlet flow area f₂ to the cross section f₁ of the cavity 4 at the place of minimum distance between the forming surfaces in the direction normal to the polymer flow being 0.01 to 1. The size of the flow area R₁ and R₂ of the inlet opening of the cavity 4 is determined by an equation taken from the theory nonstationary thermal conductivity: 1/R² =1/R₁ ² +1/R₂ ², or by any known relationship depending on the shape of the inlet opening.

The process of the invention is carried into effect by means of a split mold whose various structural embodiments are shown in FIGS. 2, 4 and 5.

The split mold, such as shown in FIG. 2, comprises a cover half-mold 7 and an ejector half-mold 8 attached respectively to a fixed plate 9 and a movable plate 10 of an injection-molding machine and carrying respectively a die 11 and a force plunger 12. With the half-molds brought together, the forming surfaces A and B of the die 11 and the force plunger 12, respectively, define cavities 13 and 14 depending on the number of items 15 and 16 to be produced. The cavities 13 and 14 intercommunicate via an opening 17 (or a channel) through which the polymer melt passes as it is pressed out from the cavity 13 into the cavity 14. The opening 17 serves as an outlet for the cavity 13 and as an inlet for cavity 14.

The cavity 13 is brought in communication with a gating system by means of an inlet opening 18. The cavity 14 has an outlet opening (not shown) through which passes the polymer melt as it is forced through the cavity 14.

The cavity 14 is formed with an outlet opening only in case when two strengthened articles are to be produced in the split mold. Then, the second piece of the molding will have a section 19 (FIG. 1) of the solidified polymer in this outlet opening, which is then removed.

If, however, the cavity 14 (FIG. 2) is used for producing a strengthened piece by a conventional technique, the provision of an outlet in this cavity will be unneccessary.

To ensure additional or local heating of the walls of the inlet and outlet openings, that is at the places where the polymer melt leaves one cavity for another or passes from the gating system into the cavity 13, the force plunger 12 is respectively provided with replaceable inserts 20 and 21 having heaters 22 made, for example, in the form of cylidrical electric heaters of any conventional type. The replaceable inserts 20 and 21 are isolated from the remaining part of the respective half-mold by means of heat-insulating pads 23 intended for altering the flow area of openings.

The split molds shown in FIGS. 2, 4 and 5 are equipped with conventional temperature transducers (not shown) which are located in promixity to the forming surfaces.

The melt is fed from an injection-molding machine equipped with pressure and temperature transducers. The half-molds are formed with coolant supply channels provided to cool the forming surfaces to a temperature ranging from -20° C. to +30° C. A cooling plant of any conventional type may be used as the coolant supply source; an oil thermostat will be required for temperatures over +° C.

To reduce the heat losses, heat-insulating plates 26 fabricated, for example, from asbestos cement are respectively placed between the die 11 and a flange 24 and between the force plunger 24 and a support plate 25.

The temperature of the melt at the entrance to the mold is controlled by means of a temperature transducer to an accuracy of ±1° and is regulated with the aid of thermoregulators incorporated in an injection-molding machine. The time for holding the melt under pressure is determined experimentally depending on the molding weight and corresponds to the time indicating that no further growth takes place in the molding weight.

The gating system of the mold may have central and delivery gate runners or a hot-runner system for the admission of the polymer melt with a temperature exceeding by 40° to 50° C. the polymer melting or yield temperature. Where an unheated gating system is used, the central and delivery gate runners are made to have the size R_(i) of the flow area equal or greater than the corresponding sizes R₁ and R₂ of the inlet opening. This being done to prevent the material from solidifying earlier than in the inlet opening and in the mold cavities 4 and 5. To this end, the ratio of the inlet opening size to a maximum distance δ₁ between the forming surfaces is set as 1 to 4.

The provision of heaters (hot-runner gating system) permit the temperature in the gate runners, up to the mold cavity 4, to be maintained close to the polymer melting or yield temperature. This, in turn, allows for the inlet opening to function throughout the process during which the polymer melt is forced through the mold cavities 4 and 5. In this way it becomes possible to decrease the flow area of the gate runners, with the ratio of the inlet opening size to a maximum distance δ₁ etween the forming surfaces being 0.2 to 1.

In the die mold shown in FIG. 2, the cover half-mold 7, with the exception of the die 11, has an elongated rod member 27 which in cross section is shaped as shown in FIGS. 3 and 3a. In turn, the die 11 has a channel 28 extending lengthwise of the die cavities 13, 14 and opened from the side of these cavities. Mounted for reciprocation in the channel 28 is an elongated rod member 27 having its outer surface, facing the cavity, made as a forming surface A having a longitudinally extending depression adapted to be filled with the melt in the process of molding.

The reciprocating motion of the rod member 27 is provided by an actuator of any conventional type, for example, a hydraulic cylinder, which is fixed to the plate 9 by means of a bracket 30. The rod member 27 has one of its ends connected to the actuator 29 by means of a coupling 31.

The free end of the rod member 27 has a projection 32 provided to engage the layer of the polymer material being cooled, so as to displace the latter relative to the unsolidified layer of the melt.

The items or pieces 15 and 16, for example, in the form of a blade having 3 mm in thickness, and a bar 3 mm thick and 11 mm wide, are produced in the split mould, such as shown in FIG. 2, in the following manner.

The melt of a thermoplastic material is fed under pressure at a temperature exceeding the polymer initial melting or yield temperature by 40° to 50° C. during a single stroke of a worm feeder or an injection-molding machine plunger. The melt is passed through a central gate runner or through a delivery runner and the inlet opening 18 (or a hot-runner inlet) into the cavity 13, the first as viewed in the direction of the polymer flow, wherein the piece 15 or a blade is to be formed. After the cavity 13 is completely filled with the polymer melt, the latter is then forced through the cavity 13 and the heated opening 17 into the second cavity 14 intended for molding the piece 14 in the form of a bar.

The polymer melt continues to be forced through the cavities 13 and 14 until the polymer acquires a solid state throughout the volume of the pieces 15 and 16, which state is controlled, for example, by a discontinued movement of the worm feeder, whereupon a finished product (molding) is removed from the mold. An accurate control over the travelling path and velocity of the worm feeder can be effected by a slide-wire gauge. As the melt is forced through, the rod member 27 is concurrently displaced in the direction of the material flow under the action of a hydraulic cylinder. Since the external surface of the rod member 27 serves as the forming surface A, the forced displacement of one of the forming surfaces, carried out simultaneously with the melt forcing-through operation, permits the thermoplastic material to be additionally sheared and, consequently, to be oriented over the molding cross section in the cavities 13 and 14 in the direction of the material flow.

The gating system of the die mold is designed to permit the molding cooling time to be determined by a degree of the molding strength and not by the extent of freezing of the inlet opening 18, as is the case in conventional processes. This is ensured by appropriate dimensions of the central gate, i.e. the inlet opening 18, which are made greater (by four times) or equal to the maximum distance between the forming surfaces or to the molding width, such as shown in FIG. 1. Hence, proper conditions for the flow of the polymer melt will exist as long as the free cross-sectional area is preserved in the strngthened item for the polymer to be forced through.

The above holds true for the cold-runner gating system, whereas in the case of a hot-runner system, heat conditions for the polymer flow are provided by additional heaters arranged around the openings or gate runners. Therefore, the dimensions of the inlet opening are reduced, with the ratio of the inlet opening flow area to the distance between the forming surfaces of the cavity in communication with the hot-runner gating system being 0.2 to 1.

Since the process of the invention is carried out at low temperatures of the polymer melt (exceeding by not more than 40° to 50° C. the initial melting temperature for crystallizable thermoplasts or the initial yield temperature for amorphous thermoplasts), and under high casting pressures ensuring high viscosity and low flowability of the melt, the molding pressure is established such as to exceed by 1,000 kgf/cm² that used in conventional processes. It has been found that the highest degree of strength in the articles being produced is achieved when the molding pressure is increased with a decrease in the cross-sectional area of the outlet opening 17 adapted to communicate the cavity 13 with the cavity 14.

The die mold, shown in FIG. 4, is intended for the production of strengthened hollow moldings. It comprises a top half-mold 33 and a bottom half-mold 34, which are attached to plates 35 of an injection-molding machine and which carry respectively a force plunger 36 and a die 37.

When brought together, the plunger 36 and the die 37 form two cavities 38 and 39 communicating with each other and with a gating system through an outlet opening 40 and an inlet opening 41.

Mounted for rotation in the mold cavities 38 and 39 arranged coaxially therewith is a forming mandrel 42 connected with an actuator 43 and a device 44 for removing finished moldings 45 and 46.

The top half-mold 33 and the bottom half-mold 34 incorporate heat insulating plates 47, support plates 48 and flanges 49, which are respectively connected in series with the plunger 36 and the die 37.

The movable forming mandrel 42 is rigidly connected with a crosspiece 50 secured on the fixed part of an injection-molding machine (not shown). The actuator 43 is made, for example, in the form of a hydraulic cylinder with a shaft 51 having a pinion (not shown) mounted on its end for engagement with a gear wheel 52 fitted on the mandrel 42.

The device 44 for removing the moldings 45 and 46 includes a thrust member made in the form of a washer 53 connected with the half-molds 33 and 34 by means of a lever mechanism 54. The lever mechanism 54 incorporates levers 55 articulated through one end to the washer 53 and through another one to levers 56 which are pivotally connected to the support plates 48 of the top and bottom half-molds 33 and 34, and to the crosspiece 50.

The washer 53 is mounted for reciprocation on the mandrel 42 and, with the half-molds 33 and 34 broughg apart, is thrust up against the end face of the molding 46, thereby displacing the moldings 45 and 46 relative to the mandrel 42.

The plunger 36 and the die 37 have replaceable inserts 57 intended for adjusting the flow area of the outlet opening 40. The inserts 57 are provided with heaters 58 and are isolated from the plunger 36 and die 37 by means of heat-insulating pads 59. To provide additional heating of the walls of the inlet opening 41, there are provided heaters 58 of any conventional type, uniformly arranged around the inlet opening 41.

To enable cooling of the mandrel 42, the latter has holes drilled therein in longitudinal direction to permit the passage of a coolant, and communicating with a coolant supply means 60.

The control of temperature of the plunger 36 and die 37 is effected by means of a potentiometer of any conventional type, which operable to register and record signals from temperature transducers set in different places of the mold.

This embodiment of the die mold operates as follows.

During one stroke of a worm feeder or the plunger of an injection-molding machine, the polymer melt is fed under pressure at a temperature exceeding by not more than 40° to 50° C. the polymer melting or yield temperature. The melt is charged through the inlet opening 41 into the mold cavity 38, the first as viewed in the direction of the polymer flow. Once the mold cavity 38 is filled with the melt, the level of which is controlled by a thermocouple set at the end of the cavity 38, the melt is then forced through the cavity 38 and the heated outlet opening 40 to pass into the second cavity 39. As the melt is pressed through the cavity, the forming mandrel 42 is concurrently rotated until the material acquires a solid state throughout the volume of the molding being produced. The solidification process is completed when rotation of the forming mandrel 42 or movement of the worm feeder or the injection-molding machine plunger are discontinued. The resultant moldings feature strengthened and textured structure extending both in the direction of the melt flow and radially, which is extremely important for such items as couplings for use in pipelines.

The technique of forcing the polymer melt, through the mold effected by bringing together the mold surfaces, is exemplified by means of a die mold, such as shown in FIG. 5.

This type of mold is used for the production of strengthened items, such as plates, bars and other bodies. It comprises a top half-mold 61 and a bottom half-mold 62, which are attached to plates 63 of an injection-molding machine. Mounted in the half-molds 61 and 62 are respectively a plunger 64 and a die 65 which, when brought together, form two cavities 66 and 67 communicating with each other and with a gating system through an outlet opening 68 and an inlet opening 69.

The top half-mold 61 and the bottom half-mold 62 incorporate heat-insulating plates 70, support plates 71 and flanges 72, which are connected in series with the plunger 64 and the die 65.

Located at the outlet opening 68 in the support plate 71 of the die 65 is a valve device 73 intended for closing or opening the outlet opening 68. The valve device 73 consists of a hydraulic cylinder 74 with a piston having a rod 75 connected to a gate 76 mounted for reciprocation in a shaped hole of the plunger 64.

The support plate 71 of the bottom half-mold 62 has a blind hole adapted to accommodate a piston 77 of a power cylinder with a rod made as a movable member 78 sliding toward the forming surface E of the plunger 64 and carrying the forming surface C of the die 65. The movable member 78 comes into intimate contact with the forming surface D of the die 65 over a sliding seat. The support plate 71 has holes 79 and 80 for supply and discharge of a working fluid of the power cylinder.

The flow area of the outlet opening 68 is adjusted by means of replaceable inserts 81 placed in the die 65 and provided with conventional heaters 82. The inserts 81 are isolated from the die 65 by means of heat insulating pads 83. Additional heating of the inlet opening 69 is effected with the aid of heaters 84 arranged uniformly around the opening 69.

The cooling of the movable member 78 (rod of the piston 77) is effected by means of additional channels 84 connected to a coolant supply distribution system (not shown). To reduce heat losses, the rod of the piston 77 is made up of at least two parts with a heat-insulating plate 85 positioned therebetween. This plate can be made of any suitable material, for example, asbestos cement.

The control of temperature of the plunger 64 and the die 65 is effected by means of a conventional potentiometer.

This die mold operates as in a manner similar to that of the above-described molds with the exception that the polymer melt is injected into the mold cavity, the initial volume of which is 1.5 to 3 times that of the molding. Then, after the cavity 66 is filled, the piston 77 is actuated to displace the member 78, carrying the forming surface C of the half-mold 62, toward another forming surface E until the molding being produced acquires a pressed thickness and a solid state over the entire volume thereof. As this happens, the excessive polymer melt is pressed out of the cavity 66 into the cavity 67 by means of the valve device 73 operable to open the outlet 68, or through the inlet 69 into the cylinder of an injection-molding machine.

The illustrative examples for carrying out the process of the invention and destruction tests for the moldings produced by the prior-art process and by the process according to the present invention are give in Tables 1 to 3.

                                      TABLE 1                                      __________________________________________________________________________     HIGH-DENSITY POLYETHYLENE                                                                                         Flow Area                                                                      of Inlet                                                                       Opening (mm.sup.2)                          Molding                                                                             Temperature of Melt, °C.                                                                              for Standard                                Pressure,                                                                           125       150       175       Specimens                                   kgf/cm.sup.2                                                                        Temperature of the Mold Cavity Wall, °C.                                                              3 mm thick                                  1    2  3  4   5  6  7   8  9  10  11                                          __________________________________________________________________________     900  +15                                                                               +20                                                                               +30 +10                                                                               +20                                                                               +25 -- +20                                                                               --                                                   235                                                                               240                                                                               238 225                                                                               230                                                                               228 -- 220                                                                               --  0                                                720                                                                               850                                                                               720 690                                                                               790                                                                               680 -- 670                                                                               --  0,7                                              730                                                                               960                                                                               725 680                                                                               950                                                                               680 -- 780                                                                               --  0,9                                              730                                                                               1000                                                                              730 690                                                                               980                                                                               690 -- 850                                                                               --  1,2                                              720                                                                               820                                                                               720 690                                                                               800                                                                               690 -- 750                                                                               --  2,0                                         1200 -15                                                                               +20                                                                               +45 -10                                                                               +20                                                                               +40 +15                                                                               +20                                                                               +25                                                  255                                                                               260                                                                               250 245                                                                               250                                                                               240 235                                                                               245                                                                               235 0                                                780                                                                               880                                                                               790 750                                                                               850                                                                               750 730                                                                               780                                                                               730 0,5                                              780                                                                               1250                                                                              790 750                                                                               1150                                                                              750 735                                                                               990                                                                               740 0,9                                              800                                                                               1280                                                                              800 760                                                                               1230                                                                              760 735                                                                               1080                                                                              735 1,0                                              780                                                                               1020                                                                              780 760                                                                               1010                                                                              750 740                                                                               780                                                                               730 2,0                                         1400 -20                                                                               +20                                                                               +60 -20                                                                               +20                                                                               +60 -10                                                                               +20                                                                               +50                                                  300                                                                               305                                                                               290 280                                                                               300                                                                               295 260                                                                               270                                                                               250 0                                                915                                                                               1150                                                                              920 890                                                                               1080                                                                              900 800                                                                               850                                                                               800 0,4                                              920                                                                               1550                                                                              920 900                                                                               1500                                                                              900 880                                                                               1200                                                                              880 0,9                                              910                                                                               1220                                                                              920 900                                                                               1200                                                                              900 800                                                                               1010                                                                              800 2,0                                         3000 -20                                                                               +20                                                                               +105                                                                               -20                                                                               +20                                                                               +115                                                                               -20                                                                               +20                                                                               +125                                                 450                                                                               610                                                                               430 420                                                                               600                                                                               410 390                                                                               540                                                                               370 0                                                1200                                                                              1500                                                                              1200                                                                               1200                                                                              1400                                                                              1200                                                                               1000                                                                              1100                                                                              1000                                                                               0,2                                              1200                                                                              2000                                                                              1200                                                                               1200                                                                              2000                                                                              1200                                                                               1100                                                                              1500                                                                              1100                                                                               0,5                                              1200                                                                              1600                                                                              1200                                                                               1200                                                                              1500                                                                              1200                                                                               1000                                                                              1300                                                                              1000                                                                               0,8                                         __________________________________________________________________________

                                      TABLE 2                                      __________________________________________________________________________     PROPYLENE                                                                                                        Flow Area                                                                      of Inlet                                                                       Opening                                                                        (mm.sup.2) for                               Molding                                                                             Temperature of Melt, °C.                                                                             Standard                                     Pressure,                                                                           160       180       200      Specimens                                    kgf/cm.sup.2                                                                        Temperature of the Mold Cavity Wall, °C.                                                             3 mm thick                                   __________________________________________________________________________     900  +20                                                                               +20                                                                               +25 +10                                                                               +20                                                                               +35 +15                                                                               +20                                                                               +25                                                  260                                                                               280                                                                               300 240                                                                               250                                                                               260 235                                                                               240                                                                               235                                                                               0                                                 400                                                                               550                                                                               400 400                                                                               500                                                                               400 350                                                                               400                                                                               400                                                                               0,7                                               400                                                                               600                                                                               450 400                                                                               600                                                                               400 350                                                                               400                                                                               400                                                                               0,9                                               400                                                                               800                                                                               500 400                                                                               700                                                                               400 350                                                                               600                                                                               400                                                                               1,2                                               400                                                                               700                                                                               500 400                                                                               600                                                                               400 350                                                                               450                                                                               400                                                                               2,0                                          1200 -15                                                                               +20                                                                               +45 -10                                                                               +20                                                                               +40 +10                                                                               +20                                                                               +30                                                  280                                                                               310                                                                               350 260                                                                               290                                                                               320 250                                                                               260                                                                               270                                                                               0                                                 450                                                                               600                                                                               500 400                                                                               550                                                                               500 400                                                                               500                                                                               400                                                                               0,5                                               450                                                                               800                                                                               500 500                                                                               700                                                                               500 400                                                                               550                                                                               400                                                                               0,9                                               450                                                                               900                                                                               550 600                                                                               800                                                                               500 400                                                                               650                                                                               400                                                                               1,0                                               450                                                                               700                                                                               500 550                                                                               650                                                                               500 400                                                                               570                                                                               400                                                                               2,0                                          1400 -20                                                                               +20                                                                               +60 -20                                                                               +20                                                                               +60 -10                                                                               +20                                                                               +50                                                  350                                                                               380                                                                               400 370                                                                               400                                                                               290 310                                                                               350                                                                               290                                                                               0                                                 590                                                                               1040                                                                              650 650                                                                               910                                                                               620 520                                                                               720                                                                               520                                                                               0,4                                               590                                                                               1200                                                                              720 780                                                                               1040                                                                              630 540                                                                               850                                                                               530                                                                               0,9                                               590                                                                               910                                                                               650 720                                                                               850                                                                               620 520                                                                               740                                                                               520                                                                               2,0                                          3000 -20                                                                               +20                                                                               +140                                                                               -20                                                                               +20                                                                               +150                                                                               -20                                                                               +20                                                                               +60                                                  530                                                                               760                                                                               600 510                                                                               740                                                                               600 440                                                                               620                                                                               520                                                                               0                                                 770                                                                               1400                                                                              850 850                                                                               1200                                                                              800 680                                                                               930                                                                               680                                                                               0,2                                               770                                                                               1600                                                                              930 1010                                                                              1400                                                                              820 700                                                                               1100                                                                              700                                                                               0,5                                               770                                                                               1200                                                                              850 930                                                                               1100                                                                              800 680                                                                               960                                                                               680                                                                               0,8                                          __________________________________________________________________________

                                      TABLE 3                                      __________________________________________________________________________     POLYAMIDE                                                                                                         Flow Area                                                                      of Inlet                                                                       Opening                                                                        (mm.sup.2) for                              Molding                                                                             Temperature of Melt, °C.                                                                              Standard                                    Pressure,                                                                           180       190       200       Specimens                                   kgf/cm.sup.2                                                                        Temperature of the Mold Cavity Wall, °C.                                                              3 mm thick                                  __________________________________________________________________________     900  +5 +20                                                                               +35 +10                                                                               +20                                                                               +30 +15                                                                               +20                                                                               +25                                                  310                                                                               330                                                                               290 320                                                                               350                                                                               300 330                                                                               360                                                                               320 0                                                450                                                                               770                                                                               440 430                                                                               710                                                                               420 400                                                                               600                                                                               400 0,7                                              530                                                                               840                                                                               470 490                                                                               750                                                                               460 420                                                                               670                                                                               420 0,9                                              420                                                                               720                                                                               410 420                                                                               700                                                                               400 400                                                                               600                                                                               400 1,2                                              400                                                                               650                                                                               380 380                                                                               600                                                                               370 390                                                                               530                                                                               390 2,0                                         1200 0  +20                                                                               +40 +5 +20                                                                               +35 +10                                                                               +20                                                                               +30                                                  320                                                                               340                                                                               300 340                                                                               360                                                                               310 350                                                                               380                                                                               330 0                                                600                                                                               920                                                                               570 560                                                                               850                                                                               510 490                                                                               770                                                                               440 0,5                                              560                                                                               860                                                                               530 500                                                                               800                                                                               470 440                                                                               720                                                                               430 0,9                                              500                                                                               820                                                                               490 490                                                                               760                                                                               450 400                                                                               660                                                                               400 1,0                                              470                                                                               760                                                                               420 430                                                                               720                                                                               400 400                                                                               540                                                                               380 2,0                                         1400 -10                                                                               +20                                                                               +50 0  +20                                                                               +40 +5 +20                                                                               +35                                                  330                                                                               350                                                                               310 350                                                                               380                                                                               320 370                                                                               400                                                                               310 0                                                780                                                                               1200                                                                              740 730                                                                               1110                                                                              660 640                                                                               1000                                                                              570 0,4                                              730                                                                               1120                                                                              690 650                                                                               1040                                                                              610 570                                                                               940                                                                               560 0,9                                              650                                                                               1070                                                                              640 640                                                                               990                                                                               590 520                                                                               860                                                                               520 2,0                                         3000 -20                                                                               +20                                                                               +160                                                                               -20                                                                               +20                                                                               +170                                                                               -20                                                                               +20                                                                               +180                                                 360                                                                               400                                                                               340 380                                                                               430                                                                               350 400                                                                               450                                                                               340 0                                                1010                                                                              1550                                                                              960 950                                                                               1450                                                                              860 830                                                                               1300                                                                              740 0,2                                              950                                                                               1450                                                                              900 850                                                                               1350                                                                              800 740                                                                               1220                                                                              730 0,5                                              850                                                                               1390                                                                              830 830                                                                               1280                                                                              760 680                                                                               1120                                                                              680 0,8                                         __________________________________________________________________________

From the above it follows that with the process of the invention it becomes possible to increase the strength of polymer in the molding 3 to 5 times and over by producing directionally oriented structure of the material; to expand the mold cooling temperature range along with an increase in the molding pressure while ensuring a required strength properties of the molding material; to improve physical and mechanical properties of thermoplastic materials and to widen the scope of their application; to replace expensive and scarce polymer materials by cheap ones, such as polyethyelene styrene copolymer, etc. 

What is claimed is:
 1. A process for injection molding of a thermoplastic polymer in a mold having molding surfaces which define interconnected molding cavities, arranged in series, interconnected by inlet and outlet openings wherein at least a molding cavity in the series provides an opening for the thermoplastic polymer to exit the molding cavity which comprises: (a) forcing a fluid polymer to flow serially through said interconnected cavities in a continuous flow under pressure, at a temperature lower than about 50° C. above the melt point or flow point of the polymer, through an inlet opening into the cavities in series; (b) maintaining the polymer under a pressure of from about 900 to 3000 Kg f/cm² until the polymer solidifies in the cavities wherein the temperature of the molding surfaces are maintained at a temperature from about -20° C. to the melt or flow point of the polymer.
 2. The process of claim 1 wherein, the polymer is heated in the outlet openings which interconnect the molding cavities.
 3. A process according to claim 1 wherein as the fluid thermoplastic polymer is forced through the interconnected cavities, at least one of the molding surfaces is moved in a direction along and a direction opposite to the flow of the polymer at least until the polymer solidifies.
 4. A process according to claim 1, wherein as the fluid thermoplastic polymer is forced through the interconnected cavities, at least one of the molding surfaces is caused to rotate until at least the polymer solidifies. 